Pultrusion is a continuous process for the manufacturing of composite materials with a constant cross-section, and is a commonly used method in the production of composite fibers. A typical pultrusion process flow diagram in which the steps are practiced horizontally is represented by FIG. 2. In the standard pultrusion process the reinforcement materials like fibers or braided strands (1) are pulled horizontally through creel guide and tension rollers (2), then fiber strands are impregnated with resin (3), possibly followed by a separate preforming system, and pulled through a heated stationary die (4) where the resin undergoes polymerization. The impregnation is either done by pulling the reinforcement through a bath or by injecting the resin into an injection chamber which typically is connected to a die. Next, the impregnated fiber is pulled through a surface shaping station (5) and cured inside a preheated curing chamber (6). Linear horizontal movement of the pultruded member is provided by a pull mechanism (7). In a final stage the pultruded member is cut to required lengths at cutting station (8).
Some known pultrusion processes include those described in the following U.S. Patents, including U.S. Pat. No. 3,895,896; in which articles are made by folding ribbons of fiber mat while dry, over and around one or more mandrels having an exterior configuration which delineates a hollow cavity, or cavities, extending longitudinally through the article. U.S. Pat. No. 4,194,873, describes a fiber reinforced pultruded rod-like reinforcing element that includes at least one groove and/or protrusion along its length, with the continuous reinforcing fibers in the protrusion or surrounding the groove generally conforming to the pattern of the outer surface of the rod in a helical pattern. U.S. Pat. No. 4,289,465 and divisional patent U.S. Pat. No. 4,296,060, describe processes in which twisted pultruded fiber reinforced rods are formed. In forming the rods, fibers are coated with a resin, drawn through a shaping die and then after at least partially curing, the rod is simultaneously twisted and pulled through a coater by opposed pulling surfaces that rotate as they pull to twist the rod. U.S. Pat. No. 4,938,823 describes a method for the manufacture of fiber reinforced plastic articles including the steps of pultruding a first profile through a die and applying a thermoplastic resin to the first profile to form a second profile bonded integrally to the first profile. U.S. Pat. No. 4,752,513 describes resin reinforcing composite mats of continuous strands for use in pultrusion processes. The pultruded parts are characterized by having a reinforcement of mats and rovings with the reinforcing mats and rovings being distributed throughout the parts. U.S. Pat. No. 6,800,164 describes composite reinforcing rods formed by using a mandrel or plastic tubing core to form a hollow shape of the composite materials to get an externally threaded composite tubing. U.S. Pat. No. 6,893,524 describes a method including wetting fibers with a resin capable of being cured by at least two different cure treatments, and at least partially curing the resin by subjecting the resin to a first curing treatment and at least partially curing the resin by subjecting the resin to a second curing treatment. A plurality of fibers are located adjacent to each other so that a plurality of valleys are formed between the plurality of fibers along an outer side of the reinforcement. The resin is cured to retain the valleys in the outer side of the reinforcement. U.S. Pat. No. 8,123,887 describes a continuous method for making oriented fiber composites for use in thin materials. Each of U.S. Pat. Nos. 3,895,896, 4,194,873, 4,289,465, 4,296,060, 4,938,823, 4,752,513, 6,800,164, 6,893,524, and 8,123,887 discussed above is incorporated herein by reference in its entirety.
None of these references address the problems discovered by the present inventors and addressed in this disclosure. Tensile testing of composite fibers from different manufacturers revealed that breaking loads for fibers, even those within the same batch, may vary by more than a factor of 2 as shown in FIG. 1B. It can be seen in FIG. 1B, for example, that a tensile test of one hundred samples from the same batch exhibited an average tensile strength of the fibers of about 256.8 ksi (kilopounds per square inch) with a variability of about 37% above and below the average line. Similarly tested steel fibers did not exhibit this variability. Although basalt fibers, for example, offer certain advantages over steel reinforced concrete, concrete reinforce with conventionally produced composite fibers may be more prone to cracking because of the variability in the tensile strength of the composite fibers.
A careful study of each stage of the pultrusion process revealed the occurrence of a large number of gas microbubbles arising at the stage of impregnation of the fiber strands with resin. Fiber strands are composed of thousands of filaments coated with sizing film. Sizing film is well known term in the art and can be described as a sprayed film that is applied to filaments as they are formed leaving a die, for example. Sizing typically includes a film forming agent such as a silane and a coupling agent, although many more complex chemistries can be used for certain products. The purpose of the sizing is to protect and lubricate the fibers and to hold the fibers together. Images of filaments taken by a Scanning Electron Microscope show that the sizing film for conventionally produced fibers is not flawless, the filaments' surfaces are uneven and heterogeneous. Molecules of gases are very easily entrained by these surface irregularities and are present within the bundle of strands when added to a resin bath. As the fiber bundles are submerged in the resin the gas molecules remain trapped inside the bundle. None of the further actions that are typical of pultrusion processes such as heating, squeezing, curing, etc. can remove these gas bubbles from the fibers. The viscosity of the resin is so high that the gas bubbles remain in the pultruded member in its finished form. These trapped gas bubbles result in a weakening of the mechanical strength of the pultruded member.
Therefore, there remains a need for systems and methods that allow continuous production of uniform composite fibers without irregularities in the form of trapped gas bubbles or microbubbles.